1. Technical Field
The technology described herein relates generally to wireless networking. More particularly, the technology relates to the transmission and reception of symbols (such as symbols in Orthogonal Frequency Division Multiple Access (OFDMA) frames) that include pilots transmitted using pilot tones.
2. Description of the Related Art
Wireless LAN (WLAN) devices are currently being deployed in diverse environments. Some of these environments have large numbers of access points (APs) and non-AP stations in geographically limited areas. In addition, WLAN devices are increasingly required to support a variety of applications such as video, cloud access, and offloading. In particular, video traffic is expected to be the dominant type of traffic in many high efficiency WLAN deployments. With the real-time requirements of some of these applications, WLAN users demand improved performance in delivering their applications, including improved power consumption for battery-operated devices.
A WLAN is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” A series of standards have been adopted as the WLAN evolved, including IEEE Std 802.11™-2012 (March 2012) (hereinafter, IEEE Std 802.11). The IEEE Std 802.11 was subsequently amended by IEEE Std 802.11ae™-2012, IEEE Std 802.11aa™-2012, IEEE Std 802.11ad™-2012, and IEEE Std 802.11ac™-2013 (hereinafter, IEEE 802.11ac).
Recently, an amendment focused on providing a high efficiency (HE) WLAN in high-density scenarios is being developed by the IEEE 802.11ax task group. The 802.11ax amendment focuses on improving metrics that reflect user experience, such as average per station throughput, the 5th percentile of per station throughput of a group of stations, and area throughput. Improvements will be made to support environments such as wireless corporate offices, outdoor hotspots, dense residential apartments, and stadiums.
An HE WLAN supports Orthogonal Frequency Division Multiple Access (OFDMA) communications. In the OFDMA communications, an Access Point (AP) may communicate simultaneously with a plurality of stations by allocating respective Resource Units (RUs) (that is, groups of sub channels) within the stations.
An HE WLAN also supports the use of longer symbols in data fields of an HE frame. For example, while a preamble of the HE frame may include Orthogonal Frequency Division Multiplexing (OFDM) symbols have respective durations, exclusive of Cyclic Prefixes (CPs), of 3.2 microseconds, a data field of the HE frame may include OFDM symbols have respective durations, exclusive of CPs, of 12.8 microseconds.
A duration of an OFDM symbol may be determined according to a number of input elements of a Fourier Transform (FT) or Inverse Fourier Transform (iFT) respectively used to decode or generate the OFDM symbol. An OFDM symbol having a duration, exclusive of CP, of 3.2 microseconds in a 20 MHz bandwidth may be generated using an iFT having 64 input elements (i.e. a 64-element iFT) and decoded using a FT having 64 input elements (i.e. a 64-element FT). An OFDM symbol having a duration, exclusive of CP, of 12.8 microseconds in a 20 MHz bandwidth may be generated using an iFT having 256 input elements (i.e. a 256-element iFT) and decoded using a FT having 256 input elements (i.e. a 256-element FT). A number of input elements of an FT or iFT may be referred to as a size of the FT or iFT.
Pilots are used in 802.11 systems for performing channel estimation and for performing carrier frequency offset (CFO) tracking. Pilots used for channel estimation may be included in a training field, such as a Long Training Field (LTF).
CFO may occur, for example, because of a frequency mismatch between oscillators of a transmitter and a receiver or because of the Doppler Effect due to relative motions of the transmitter and receiver. Even if the channel state does not change over a duration of a received frame, a residual CFO may changeover the duration. Because the CFO may change during the duration, pilots for CFO tracking may be included in symbols of data fields. Such pilots may be carried by pilot tones located at pilot tone positions of the symbols.
Ideally, pilots are included in all OFDM symbols, and span the entire frequency bandwidth of the transmitted signal so that CFO tracking performance may be improved by the inclusion of frequency diversity. The positioning of pilots tones carrying the pilots may vary between symbols in training fields and symbols in data fields, and between symbols generated using different Fourier Transform (FT) sizes.